Proton Transfer Accompanied by the Oxidation of Adenosine

Abstract

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Despite numerous experimental and theoretical studies, the proton transfer accompanying the oxidation of 2′-deoxyadenosine 5′-monophosphate 2’-deoxyadenosine 5’-monophosphate (5’-dAMP, A) is still under debate. To address this issue, we have investigated the oxidation of A in acidic and neutral solutions by using transient absorption (TA) and time-resolved resonance Raman (TR 3 ) spectroscopic methods in combination with pulse radiolysis. The steady-state Raman signal of A was significantly affected by the solution pH, but not by the concentration of adenosine (2–50 mm). More specifically, the A in acidic and neutral solutions exists in its protonated (AH + (N1+H + )) and neutral (A) forms, respectively. On the one hand, the TA spectral changes observed at neutral pH revealed that the radical cation (A .+ ) generated by pulse radiolysis is rapidly converted into A . (N6−H) through the loss of an imino proton from N6. In contrast, at acidic pH (<4), AH .2+ (N1+H + ) generated by pulse radiolysis of AH + (N1+H + ) does not undergo the deprotonation process owing to the pK a value of AH .2+ (N1+H + ), which is higher than the solution pH. Furthermore, the results presented in this study have demonstrated that A, AH + (N1+H + ), and their radical species exist as monomers in the concentration range of 2–50 mm. Compared with the Raman bands of AH + (N1+H + ), the TR 3 bands of AH .2+ (N1+H + ) are significantly down-shifted, indicating a decrease in the bond order of the pyrimidine and imidazole rings due to the resonance structure of AH .2+ (N1+H + ). Meanwhile, A . (N6−H) does not show a Raman band corresponding to the pyrimidine+NH 2 scissoring vibration due to diprotonation at the N6 position. These results support the final products generated by the oxidation of adenosine in acidic and neutral solutions being AH .2+ (N1+H + ) and A . (N6−H), respectively